Ballasts for operating fluorescent lamps in a so-called “instant start” mode of operation are widely known in the art. Instant start operation necessitates that the ballast provide a very high voltage (e.g., 650 volts RMS or so) for properly igniting the lamps.
A popular circuit topology for realizing instant start ballasts includes a current-fed self-oscillating half-bridge type inverter and an isolated parallel resonant output circuit. This topology has been widely utilized because of its reliability and cost-effectiveness.
An example of an existing ballast 10 that incorporates the aforementioned circuit topology is described in FIG. 1. During operation, ballast 10 provides a relatively high voltage (e.g., 650 volts RMS) between output connection 210 and each of output connections 202,204 for igniting lamps 32,34. Once lamps 32,34 are ignited, ballast 10 provides a magnitude-limited current for steady-state operation of lamps 32,34.
For applications involving T8 type lamps, the high ignition voltage that is provided by ballast 10 does not present any substantial difficulties in connection with applicable regulatory or safety requirements concerning the maximum voltages that may exist between the lamp sockets (which are connected to output connections 202,204,210) and the housing of the lamp fixture (which is connected to earth ground). Thus, the circuit topology described in FIG. 1 has been widely utilized for powering T8 type lamps without violating any regulatory/safety requirements concerning maximum socket-to-fixture voltage.
For applications involving lower wattage T5 type lamps (e.g., 28 watt, 35 watt, and 40 watt T5 lamps), on the other hand, the situation is different. Lower wattage T5 lamps typically require an ignition voltage that is at about the same level (e.g., 650 volts RMS) required for T8 lamps. However, the requirements pertaining to lamp fixtures for T5 lamps typically dictate that the socket-to-fixture voltage must not exceed 430 volts RMS (i.e., 430 VAC). Unfortunately, ballast 10 is generally incapable of satisfying the dual, and conflicting, requirements of a high ignition voltage (e.g., 650 volts RMS) and a maximum socket-to-fixture voltage (i.e., less than 430 volts RMS) for applications involving T5 (or smaller diameter) lamps.
FIG. 2 depicts a simplified circuit model (i.e., an equivalent circuit) that is helpful in understanding the operation of ballast 10 with regard to socket-to-fixture voltage. The arrangement is modeled as a 650 VAC voltage source that is connected between output connections 202,210 (which, in turn, are coupled to corresponding sockets in lamp fixture 40). Output connections 202,210, and their corresponding sockets, are normally taken to be electrically isolated from lamp fixture 40 and earth ground 80. In an ideal situation, the 650 VAC voltage source is evenly distributed, such that the socket-to-fixture voltage (i.e., the voltage that exists between each lamp socket and earth ground) does not exceed about 325 VAC, which is well within the required limit of 430 VAC. In an actual ballast, however, output circuit 200 is typically significantly unbalanced due to the presence of leakage impedances and stray capacitances to earth ground. Thus, in an actual implementation of ballast 10, the 650 VAC is generally not evenly distributed, and the socket-to-fixture voltage involving at least one of the lamp sockets is likely to exceed the required limit of 430 VAC. Moreover, the structure of output circuit 200 in ballast 10 is inherently asymmetrical due to the fact that ballasting capacitors 252,254 are both coupled to a single end of secondary winding 234; that asymmetry has the effect of further contributing to the imbalance in the voltage distribution between the two lamp sockets (with respect to earth ground) under no load conditions (i.e., prior to ignition of the lamps 32,34, or when one or more of the lamps 32,24 is inoperative or removed from the lamp fixture).
As previously mentioned, the circuit topology of ballast 10 is known to be reliable and cost-effective. Although the socket-to-fixture voltage is typically not an issue when ballast 10 is used to power T8 lamps, it is a problem for applications involving T5 lamps (or smaller diameter lamps). Thus, a need exists for an approach by which the circuit topology of ballast 10 may be adapted for powering lower wattage T5 lamps, while satisfying the more stringent socket-to-fixture voltage limitations for T5 lamp fixtures. Accordingly, a ballast that realizes such an approach in an effective, energy-efficient, and economical manner would represent a considerable advance over the prior art.